1. Field of the Invention
The present invention relates to a transparent conductive layer forming coating liquid for preparing a transparent conductive layer on a transparent substrate. The present invention particularly relates to a transparent conductive layer forming coating liquid that forms a transparent conductive layer with an excellent transmission profile in the range of visible light, weather resistance and provision of an excellent anti-reflection effect and electric field shielding affect in the case that a transparent conductive layered structure on which the above-mentioned transparent conductive layer is formed is used in front panels of display devices such as Braun tubes (CRTs), plasma display panels (PDPs), vacuum fluorescent displays (VFDs), liquid crystal displays (LCDs), and so on.
2. Description of the Related Art
Some conditions are required for a cathode ray tube (also called a Braun tube as mentioned above: CRT) now being used for computer displays, and so on. It must be easy to see the display screen 3 in order to prevent visual fatigue, as well as prevent deposition of dust and electric shock induced by the electrostatic charge on the CRT screen, etc. Furthermore, in addition to the requirements, there has recently been concern over the detrimental effects of low-frequency electromagnetic waves generated by CRTs on the human body and there is a demand for CRTs with which there is no leakage to the outside of such electromagnetic waves.
Further, in recent years, the problems of the above-stated electrostatic charge and leakage of electromagnetic waves are also pointed out in plasma display panels (PDP) used for wall-hung TVs, and so on, as in CRTs.
It is possible to prevent such leakage of electromagnetic waves, for example, by coating the front panel surface of a display with a transparent conductive layer.
The above-stated method for preventing the leakage of electromagnetic waves is theoretically the same as measures that have been adopted in recent years to prevent electrostatic charging. However, conductivity of the above-mentioned transparent conductive layer must be much higher than that of conductive layers that are formed to prevent electrostatic charging (surface resistance of approximately 108 to 1010 xcexa9/xe2x96xa1, ohm per square).
That is, in CRTs, a transparent conductive layer with at least as low a resistance as 106 xcexa9/xe2x96xa1 or less, preferably 5xc3x97103 xcexa9/xe2x96xa1 or less, and more preferably 103 xcexa9/xe2x96xa1 or less is preferred for prevention of leakage of an electric field (electric field shielding). On the other hand, in PDPs, 10 xcexa9/xe2x96xa1 or less is demanded, for instance.
Moreover, several suggestions have been made thus far to take measures for the above-mentioned electric field shielding. For instance, in CRTS, there are proposals having been suggested, such as,
(1) a method wherein a coating liquid for forming a transparent conductive layer in which conductive oxide microparticles such as indium tin oxide(ITO) and so on, or metal microparticles dispersed in a solvent is applied to the front glass (a front panel) of a CRT and dried and then baked at a temperature of approximately 200xc2x0 C. for forming the above-stated transparent conductive layer,
(2) a method for forming a transparent conductive tin oxide layer (a Nesa layer) on a front glass (a front panel) by a high temperature chemical vapor deposition (CVD) method of tin chloride, and
(3) a method for forming a transparent conductive layer on a front glass (a front panel) by sputtering indium tin oxide, titanium oxynitride and so on.
Also in PDPs, several methods have been proposed, such as,
(4) a method for forming a transparent conductive film on the above-stated front panel by sputtering metals such as silver and so on, and
(5) a method for forming a conductive film by setting an conductive mesh made by metal or metal-coated fibers on a front panel at the main device body side of the front panel in PDPs.
However, there are some problems in the method (5) in PDP, that is, although low surface resistance is obtained by using a conductive mesh, the transmittance gets lower and moire occurs. Furthermore, the manufacturing processes for forming a conductive layer are complicated and cost rises accordingly.
On the other hand, the method shown in (1) in CRTs is very simple when compared to other methods of forming a transparent conductive layer such as a CVD method or sputtering method shown in (2) to (4), and has a low production cost. As a result thereof, the method (1) that uses a coating liquid for forming a transparent conductive layer is a very useful method not only in the above-stated CRTs but also in PDPs.
However, in the method shown in (1) that employs conductive oxide microparticles such as indium tin oxide (ITO) and so on, as a coating liquid for forming a transparent conductive layer, surface resistance of the film that is obtained is high at 104 to 106 xcexa9/xe2x96xa1, which was not sufficient for blocking leakage of an electric field.
On the other hand, when compared to coating liquids that use ITO, a film with somewhat lower transmittance, but also low resistance of 102 to 103 xcexa9/xe2x96xa1, is obtained with coating liquids for forming transparent conducive layers that employ metal microparticles, and this will probably be the promising method of the future.
Moreover, the metal microparticles that are used in the above-mentioned coating liquid for forming the above-mentioned transparent conductive layer are limited to noble metals, such as silver, gold, platinum, rhodium, palladium, etc., that rarely oxidize in air, as shown in Japanese Patent Applications Laid-Open No. H 8-77832 and Laid-Open No. H 9-55175. This is because if microparticles of a metal other than a noble metal, such as iron, nickel, cobalt, etc., are used, an oxide film is invariably formed on the surface of such metal microparticles in an air atmosphere and good conductivity cannot be obtained as a transparent conductive layer.
Moreover, on the other hand, in order to make the display screen easy to see, anti-glare treatment is performed on the front panel surface to prevent reflection on the screen, for example, in CRTs.
This antiglare treatment is performed by the method whereby fine irregularities are made in the surface in order to increase diffused reflection at the surface, but it cannot be said that this method is a very desirable method because when used, resolution decreases and picture quality drops.
Consequently, it is preferred that antiglare treatment be performed by the interference method whereby the refractive index and film thickness of the transparent film be controlled so that there is destructive interference of the incident light by the reflected light.
A two-layered film structure wherein optical film thickness of film with a high refractive index and film with a low refractive index has been set at xc2xcxcex and xc2xcxcex, or xc2xdxcex and xc2xcxcex, respectively, is usually used in order to obtain this type of low-reflection effect of the interference method, and film consisting of the above-mentioned indium tin oxide (ITO) microparticles is also used as this type of film with a high refractive index.
Furthermore, of the optical constant (n-ik, n: refractive index, i2=xe2x88x921, k: extinction coefficient) of metals, the value of n is small, but the value of k is very large, and therefore, even if a transparent conductive layer consisting of metal microparticles is used, the same anti-reflection activity induced by interference of light as seen with ITO (film with a high refractive index) is obtained with the two-layered film structure.
Besides, as for the transparent conductive layered structure wherein the transparent conductive layer of this kind is formed on the transparent substrate, a specific feature is in recent years required to enhance contrast of pictures by controlling the transmittance to be set in the prescribed range (40 to 75%) less than 100% in order to make the display panel easier to see, in addition to some other features such as the above-stated excellent conductivity and low reflectance. And in this case, blending color pigment microparticles and so on with the above-stated coating liquid for forming a transparent conductive layer is also performed.
Since the noble metal microparticles, naturally are not transparent to visible lights, the conductive film to which noble metal microparticles are applied is preferably the one on which the least possible amount of noble metal microparticles forms conductive paths efficiently in the transparent conductive layer, in order to obtain both high transmittance and low resistance in the above-stated transparent conductive layer.
Moreover, in the general coating liquid for a transparent conductive layer, comprising, as main components, a solvent and noble metal microparticles, the noble metal microparticles tend to aggregate compared with oxide microparticles and during the process for forming a film in which a coating liquid for forming a transparent conductive layer is applied and dried, microparticles aggregate to a certain extent inevitably. Accordingly, the transparent conductive film gained by applying a coating liquid for forming a transparent conductive layer has a structure in which fine holes are introduced into the conductive layer of noble metal microparticles, that is, a meshy (network) structure (see the descriptions in Industrial Materials (Kogyo Zairyo) Vol.44, No.9, 1996, pp68-71, Japanese Patent Applications Laid-Open No. H 9-115438, Laid-Open No. H 10-1777, Laid-Open No. H 10-142401, Laid-Open No. H 10-182191 and so on). When such meshy structure is formed, a transparent conductive layer with low resistance and high transmittance is obtained. It is supposed that this is because, while the meshy structure comprising metal microparticles has a function as conductive paths, the holes formed in the meshy structure have a function for enhancing the light transmittance.
In the case that the conventional coating liquid for forming a transparent conductive layer is applied, forming the transparent conductive layer having a meshy structure is a certain extent possible, as stated above. However it is actually difficult to control the aggregation occurring during the process for forming a film in which a coating liquid for forming a transparent conductive layer is applied and dried, and a failure in this control might cause the following film defects.
For example, when using the conventional coating liquid for forming a transparent conductive layer wherein a binary system solvent comprising ethanol which is an organic solvent having a low boiling point (the boiling point is lower than 100xc2x0 C.) and water, or a solvent, to the system of which a small amount (15 wt % or less) of an organic solvent having a high boiling point (the boiling point is 100xc2x0 C. or higher) is added, is employed, it is found that a developed meshy structure can be formed easily in the transparent conductive film that is obtained. This is probably because of high surface tension of water, since an organic solvent having a low boiling point (ethanol) volatilizes faster than water in the process wherein a coating liquid for forming a transparent conductive layer is applied to a substrate and dried, so that a large amount of water may remain in the coated film just before drying. However, such coating liquid for forming a transparent conductive layer is very sensitive to traces of wiping when cleaning a substrate or spots on a substrate (oil spots, for example) because of a large amount of water remaining in the coated film just before drying, and in addition, drying speed of the coating liquid is too high because the liquid contains a large amount of an organic solvent having a lower boiling point than water. As a result thereof, in the case of forming a film with the coating liquid for forming a transparent conductive layer by spin coating, there is a problem that defects of a film occur in which radial striations (streaky unevenness formed radially from the center of a substrate toward outside) and corner unevenness (shading unevenness formed in the four corners of a substrate) are more induced.
In this case, it is possible to improve the condition by using a large amount of an organic solvent having a high boiling point (the boiling point is 100xc2x0 C. or higher) as a coating liquid for forming a transparent conductive layer, because drying speed of the coating liquid can be controlled slower thereby. However, in this case, the above-stated meshy structure could not be sufficiently gained or another defect in a film (occurrence of fine aggregates all over the film) was caused by over-aggregation of noble metal microparticles.
In addition, in order to form the above-mentioned meshy structure more positively, it is proposed in Japanese Patent Applications Laid-Open No. 2000-124662 to use a coating liquid for forming a transparent conductive layer including metal microparticles that are made to aggregate in a concatenate manner beforehand. However, in this coating liquid for forming a transparent conductive layer, since the aggregates of metal microparticles are formed in advance, a filter tends to be clogged during the process of filtration of the coating liquid for forming a transparent conductive layer, which is carried out before forming a film. And besides, there was a problem that the above-mentioned defect in a film was caused by over-aggregation of noble metal microparticles, just as stated above.
The present invention focuses on such problems, its object being to present a transparent conductive layer forming coating liquid capable of forming a more developed meshy structure easily, having such characteristics as high transmittance, low resistance, low reflectance and high strength, and moreover, capable of forming a transparent conductive layer with few defects in a film.
That is, the invention of claim 1 resides in a transparent conductive layer forming coating liquid for forming a transparent conductive layer on a transparent substrate, comprising, as its main components, a solvent and noble metal microparticles with a mean particle diameter of 1 to 100 nm dispersed in the solvent,
wherein the solvent comprises 0.005 to 1.0 wt % of formamide (HCONH2).
Moreover, the invention of claim 2 resides in a transparent conductive layer forming coating liquid according to claim 1, wherein the solvent comprises an organic solvent being compatible with water and having a boiling point of 100 to 190xc2x0 C., 1 to 50 wt % of water, and monohydric alcohol containing 5 carbon atoms or less or/and ketone containing 6 carbon atoms or less.
The invention of claim 3 resides in a transparent conductive layer forming coating liquid according to claim 1 or 2, wherein the noble metal microparticles are any of: noble metal microparticles selected from gold, silver, platinum, palladium, rhodium, and ruthenium; alloy microparticles of these noble metals; or noble metal-coated silver microparticles the surface of which is coated with these noble metals other than silver.
The invention of claim 4 resides in a transparent conductive layer forming coating liquid according to claim 3, wherein the noble metal-coated silver microparticles are silver microparticles coated with gold or platinum only or a composite of gold and platinum.
The invention of claim 5 resides in a transparent conductive layer forming coating liquid according to claim 4, wherein the coated amount of gold or platinum only or a composite of gold and platinum in the noble metal-coated silver microparticles is set in the range from 5 to 1900 parts by weight to 100 parts by weight of silver.
Next, the invention of claim 6 resides in a transparent conductive layer forming coating liquid according to any of claims 1 through 5, including color pigment microparticles.
The invention of claim 7 resides in a transparent conductive layer forming coating liquid according to claim 6, wherein the color pigment microparticles are one or more types of microparticles selected from carbon, titanium black, titanium nitride, composite oxide pigment, cobalt violet, molybdenum orange, ultramarine, Prussian blue, quinacridone pigment, anthraquinone pigment, perylene pigment, isoindolinone pigment, azo pigment, and phthalocyanine pigments
The present invention of claim 8 resides in a transparent conductive layer forming coating liquid according to any of claims 1 through 7, including an inorganic binder.